diff --git a/.gitignore b/.gitignore
index 5b3c3b1c15720..2948963310456 100644
--- a/.gitignore
+++ b/.gitignore
@@ -150,6 +150,11 @@ docs/superpowers/*
 .update-incomplete
 .update-incomplete.lock
 
+# Installer-written method stamp in the managed checkout root (scripts/install.sh).
+# Runtime metadata only — never a code change. Ignore so `git status` stays clean
+# and `hermes update`'s untracked autostash does not treat it as a local edit (#66189 / #54855).
+/.install_method
+
 # Tool Search live-test harness output — non-deterministic model transcripts,
 # regenerated by scripts/tool_search_livetest.py. Never an artifact of the repo.
 scripts/out/
diff --git a/tests/hermes_cli/test_update_autostash.py b/tests/hermes_cli/test_update_autostash.py
index be1a5f1acf4c0..5146320482187 100644
--- a/tests/hermes_cli/test_update_autostash.py
+++ b/tests/hermes_cli/test_update_autostash.py
@@ -901,3 +901,53 @@ def test_bootstrap_marker_not_autostashed_by_update(tmp_path):
         ["git", "status", "--porcelain"], cwd=tmp_path, capture_output=True, text=True
     ).stdout
     assert ".hermes-bootstrap-complete" not in status
+
+
+def test_install_method_marker_not_autostashed_by_update(tmp_path):
+    """#66189: the installer ``.install_method`` stamp must be git-ignored so
+    ``hermes update``'s ``git stash push --include-untracked`` does not sweep it
+    into an autostash on every run.
+
+    ``scripts/install.sh`` writes ``$INSTALL_DIR/.install_method`` as runtime
+    metadata; it is a sibling of ``.hermes-bootstrap-complete`` /
+    ``.update-incomplete`` and must be ignored the same way. Behavioral +
+    hermetic: adopt the project's real ``.gitignore`` (the contract under test),
+    drop the marker, and confirm the exact stash invocation the updater uses
+    leaves it untouched.
+    """
+    import shutil
+    import subprocess
+
+    if shutil.which("git") is None:
+        pytest.skip("git not available")
+
+    repo_gitignore = Path(hermes_main.__file__).resolve().parents[1] / ".gitignore"
+
+    def git(*args):
+        return subprocess.run(
+            ["git", *args], cwd=tmp_path, capture_output=True, text=True, check=True
+        )
+
+    git("init", "-q")
+    git("config", "user.email", "t@example.com")
+    git("config", "user.name", "t")
+    (tmp_path / ".gitignore").write_text(repo_gitignore.read_text())
+    (tmp_path / "tracked.txt").write_text("x\n")
+    git("add", "-A")
+    git("commit", "-qm", "init")
+
+    marker = tmp_path / ".install_method"
+    marker.write_text("managed\n")
+
+    # Exact flags used by hermes update (hermes_cli/main.py).
+    git("stash", "push", "--include-untracked", "-m", "hermes-update-autostash")
+
+    assert marker.exists(), (
+        ".install_method was swept into the update autostash — it must be listed "
+        "in .gitignore so `git stash -u` skips it (#66189)."
+    )
+    # It must not even register as a dirty/untracked change.
+    status = subprocess.run(
+        ["git", "status", "--porcelain"], cwd=tmp_path, capture_output=True, text=True
+    ).stdout
+    assert ".install_method" not in status